Sodium-beta alumina batteries (NBBs), based on a molten Na anode and β″-Al2O3 solid electrolyte (BASE), have recently gained increasing interests as an electrical energy storage device for renewable integration and grid applications, along with commercial or fleet transportation. There are mainly two types of NBBs that have been widely studied, based on the particular cathode materials. One is a sodium-sulfur (Na—S) battery of which the cathode is molten sulfur, following the cell reaction:xS+2NaNa2Sx (x=5˜3), E=2.08˜1.78 V at 350° C.  (1)
Na—S chemistry has a high theoretical energy density (˜760 Wh/kg), high energy efficiency and acceptable cycle life. The materials of the sodium-sulfur battery (i.e., alumina, sulfur and sodium) are relatively non-toxic, inexpensive and readily available. The combination of these features makes it extremely attractive compared to other technologies for grid storage such as lithium-ion, Ni-metal hydride or Pb-acid batteries. The traditional Na—S battery uses a thick solid β″-Al2O3 membrane (>1 mm) as electrolyte to separate the sulfur cathode and sodium anode, and operate at high temperatures (300˜350° C.). The high temperature is necessary for both the BASE and cathode constituents (i.e., sodium polysulfides) to achieve satisfactory electrochemical activities. However, the drawbacks of Na—S battery can include: 1) intrinsic corrosive behavior of polysulfide melts, which limits material selections for both cathode current collector and battery casing; 2) high operating temperature and open circuit cell failure mode. If the BASE is broken during battery operation, molten sulfides come in direct contact with molten sodium and the reactions between them are inherently vigorous. This can potentially result in a fire and even explosion since the battery operation temperature is close to boiling point of sulfur (440° C.). Neighboring cells can also be affected by such an event and result in severe power loss due to open circuit.
The second type of NBB is the ZEBRA battery, in which solid transition metal halides, which can include NiCl2, FeCl2 and ZnCl2, are used as active materials in the cathode. The ZEBRA battery typically needs a molten secondary electrolyte (i.e., NaAlCl4) in the cathode so as to ensure facile sodium ion transport between the BASE and solid cathode materials. The electrochemical reaction of Na—NiCl2 cells is as follows:NiCl2+2NaNi+2NaCl E=2.58 V at 300° C.  (2)
The ZEBRA battery exhibits a number of advantages over the Na—S battery, which include higher voltage, facile assembly in discharged state, less corrosive nature of cathode materials, lower operating temperature, safer cell failure mode, and better tolerance against overcharging. One notable disadvantage of the current ZEBRA technologies is the lower energy density compared to Na—S battery. Accordingly, a need exists for sodium energy storage devices exhibiting at least some advantages of both ZEBRA and Na—S technologies.